There is a continuous effort to develop pressure sensors, chemical sensors, and housing materials that are lower in cost and smaller in size, yet are characterized by high reliability, sensitivity, and linearity. For example, multiple pressure sensors, temperature, and optical sensors having sensing diaphragms, cavities, and resistive elements can be made on a single silicon wafer using semiconductor fabrication processes. In the processing of such cells, sensor elements such as the thin diaphragm of a pressure sensor are formed in a silicon wafer through preferential chemical etching. Ion implantation and diffusion techniques are then used to drive doping elements into the diaphragm, forming resistive bridge circuit elements whose electrical resistance changes with strain. As a result, deflection of the diaphragm causes a change in resistance value of the piezoresistive elements, which can then be correlated to the magnitude of the pressure applied to the diaphragm.
The use of standard single-crystal silicon wafers and standard semiconductor device fabrication processes allows many such cells to be fabricated from a single wafer, providing some economy of scale. However, silicon is susceptible to chemical attack, particle generation, and erosion by various media, particularly in applications where a high-pressure, temperature, and corrosive fluids are to be sensed, e.g., semiconductor manufacturing, long term medical implants, and automotive applications. One particularly difficult and sensitive application of integrated circuits, semiconductors and metal electrodes is in electrical or electronic device implantation in a human or animal body. Extra-cellular fluids within the body are saline, and often contain a number of other ions or other electrolytes. At body temperatures, severe and rapid corrosion may lead to rapid and untimely failure of the device. For such applications, a pressure, temperature, or chemical sensor and fluid handling devices in contact with these fluids (i.e. catheters, pumps, heat exchangers, or conduits) must also be of high chemical purity, physically rugged and resistant to the hostile environment of the sensed medium. It would be advantageous that a micromachined silicon sensor cell include some form of protection in order to realize its superior operational characteristics in the chemically hostile environment. Current methods for producing media-compatible, high-pressure sensors include enclosing a silicon sensing chip in an inert fluid, such as a silicone oil or gel, and then further separating the sensing chip from the medium to be sensed with a metal diaphragm, such that pressure must be transmitted through the metal diaphragm and fluid to the sensing chip. While achieving some of the operational advantages of silicon pressure transducer cells, the manufacturing processes for these sensors are relatively expensive and complicated.
Piezoelectric and capacitive pressure sensors are typically thin ceramic plates or diaphragms that may be coated with thick-film electrodes or bridge elements to form capacitors or strain sensing elements. However, each of these also have certain disadvantages, such as complex circuitry to detect capacitance changes, the requirement for ceramic-to-ceramic bonds, and a maximum pressure capability typically not exceeding about 1000 psi (about 7 MPa). For higher pressures, metal diaphragms have found use as the sensing element, however these diaphragms are not generally useful in corrosive aqueous solutions. Metal diaphragms generally deflect more for a given thickness and pressure than ceramic diaphragms. With metal diaphragms bridge elements or electrodes may be deposited on to a dielectric insulating layer on the metal followed by thin-film polysilicon or metal deposited on the metal diaphragm to form the bridge or electrode structures. For example, a thin-film polysilicon layer is deposited on the dielectric to form the piezoresistors of the bridge, followed by thin-film metallization to provide electrical interconnects. As is conventional, the thin-film layers are typically deposited by such processes as chemical or physical vapor deposition. The equipment necessary for these processes is expensive, and deposition rates are extremely slow and difficult to use with complex structures and large structures like housings, bellows, or conduits. Deposition of the thin-film layers requires multiple patterning, exposure, developing and stripping steps for the required thin-film photoresists and metallization, and must be carried out in a controlled environment to assure that no air borne particles are present on the surface to be coated. In addition, because such processes deposit thin-films usually no thicker than 10,000 angstroms, the surface of the metal diaphragm must be extremely smooth to avoid rough surface features penetrating through or producing discontinuities in the deposited thin films. Finally, the resistance of the resulting polysilicon thin-film piezoresistors can vary dramatically with temperature.
Typically, a sensor is contained within a chemically and mechanically protective housing. The housing essentially surrounds the sensor and any associated electronics for sensor excitation and signal processing. While this provides mechanical protection for the sensor, protection from hazardous chemicals and contaminants in the medium must also be provided. In one type of pressure sensor assembly, a silicone gel, fluorosilicone gel, or silicone oil is applied over the external surface of a pressure sensor and essentially partially fills the housing in which the pressure sensor is mounted. The gel or oil is covered with a membrane. The manufacture of these cells can be cumbersome and expensive.
Various materials have been developed to provide an electrically insulative moisture barrier over a substrate. Among the more prominent of these are aromatic polyimides such as those sold under the trade designation “Kevlar” by E.I. DuPont de Nemours, & Co. However, polyimides are highly viscous, difficult to deposit, and can easily entrain gas bubbles leading to film defects. Parylene N coatings are produced by vaporizing a di(p-xylylene) dimer, pyrolyzing the vapor to produce p-xylylene free radicals, and condensing a poly-oligomer from the vapor onto a substrate that is maintained at a relatively low temperature, typically ambient or below ambient. Parylene N is derived from di(p-xylylene), while parylene C is derived from di(monochloro-p-xylylene), and parylene D is derived from di(dichloro-p-xylylene).
Although parylenes have generally advantageous electrical, chemical resistance and moisture barrier properties, it has been found that these poly-oligomers do not adhere well to many substrate surfaces, particularly under wet conditions. Although these poly-oligomers are quite resistant to liquid water under most conditions, they are subject to penetration by water vapor which may condense at the interface between the parylene film and the substrate, forming liquid water which tends to delaminate the film from the substrate. Vapor deposited parylene films are also generally quite crystalline and are subject to cracking which may also create paths for penetration of moisture to the substrate surface. Parylene has been used to protect devices and larger substrates, or a thermally bonded fluorinated polymer casing has been used. Both have been found to offer relatively poor performance in critical applications. The parylene coatings suffer from high diffusion rates, and the thermally bonded devices, provided the devices or substrate can tolerate the processing conditions, have been known to undergo mechanical stress cracking at the bond seams.
These organic coatings may be used either alone or together with fluorosilicone gels. Fluorosilicone gels are used to protect the sensor device, wirebonds, portions of the package, and leads. Fluorosilicone gels have several disadvantages including an incompatibility with fuels (e.g., swelling).